Vegetable Pot Pie

Name: Vegetable Pot Pie
Ingredients: 1 single-crust pie dough; 4 tablespoons unsalted butter; 1 onion , chopped fine; 8 ounces cremini mushrooms , trimmed and quartered if large or halved if small; Salt and pepper; 1 sweet potato (12 ounces), peeled and cut into ½-inch pieces; 8 ounces turnips , peeled and cut into ½-inch pieces; 3 garlic cloves , minced; 1/2 teaspoon grated lemon zest plus 1 tablespoon juice; 8 ounces Swiss chard , stemmed and cut into 1-inch pieces; 2 tablespoons all-purpose flour; 2 cups vegetable broth; 1 ounce Parmesan cheese , grated (½ cup); 2 tablespoons minced fresh parsley; 1 large egg; 1 teaspoon water